Method for the continuous production of methoxyamine hydrochloride

ABSTRACT

The present invention relates to a process for the continuous preparation of methoxyamine hydrochloride by cleavage of acetone oxime methyl ether by means of hydrogen chloride and water, wherein the cleavage is carried out in a reaction column having less than 20 theoretical plates and the amount taken off at the top is set to at least 30% of the amount of feed.

This is a 371 of PCT/EP00/01638 filed Feb. 28, 2000.

The present invention relates to a process for the continuouspreparation of methoxyamine hydrochloride by cleavage of acetone oximemethyl ether by means of hydrogen chloride in the presence of water.

The prior art describes both batchwise processes (EP-A 591 798, EP-A 708082) and a continuous process (EP-A 259 850) for preparing methoxyaminehydrochloride from acetone oxime methyl ether. The advantages of acontinuous process are generally the smaller dimensions of theapparatuses, the higher degree of automation and, in particular, theincreased process capability. Industrial products produced usingcontinuous processes are therefore usually more economical than theproducts obtained from batchwise processes. Furthermore, consistentlyhigh product qualities can be achieved by means of continuous processes.

However, the continuous process known from EP-A 259 850 for the cleavageof acetone oxime methyl ether by means of hydrogen chloride to formmethoxyamine hydrochloride has the great disadvantage that a reactioncolumn having many theoretical plates is necessary to achieve highyields. Such an apparatus is costly and its operation consumes a largeamount of energy.

It is an object of the present invention to develop a continuous processwhich does not have the disadvantage described in EP-A 259 850.

We have found that this object is achieved by the process mentioned atthe outset when the cleavage is carried out in a reaction column havingless than 20 theoretical plates and the amount taken off at the top isset to at least 30% of the amount of feed.

The acetone formed in the reaction and excess hydrogen chloride andwater are taken off at the top. In the process of the invention, it isimportant that at least 30% of the amount of feed are taken off at thetop, since otherwise the acetone concentration would be too high,especially in the upper segments of the reaction column. A high acetoneconcentration has an adverse effect on the position of the reactionequilibrium and also leads to a lowering of the internal temperature inthese segments. If the cleavage of the acetone oxime methyl ether iscarried out in a reaction column having less than 20 theoretical platesand the acetone is not removed to a sufficient extent, the cleavageproduct methoxyamine hydrochloride is obtained in completelyunsatisfactory yields.

The method employed in the process of the present invention in which atleast 30% of the amount of feed are taken off at the top leads to ahomogeneous temperature distribution over the column. In general, thetemperature difference between the bottom and the top (measured on thesecond-last vertical plate) is less than 15° C. and preferably less than10° C.

The process of the present invention surprisingly makes it possible toobtain methoxyamine hydrochloride in excellent yields even in a columnhave less than 20 theoretical plates.

In the process of the present invention, the mixture taken off at thetop of the column is preferably taken off at a fixed reflux ratio offrom 2 to 15, preferably from 3 to 11. The small number of theoreticalplates and the favorable reflux ratio result in a significantly shortermean residence time of the reaction mixture in the column compared toEP-A 259 850. Surprisingly, excellent yields can be achieved in theprocess of the present invention despite the short mean residence timeof only 20 minutes. Longer residence times in the column are, however,likewise possible.

The process of the invention will be described in more detail below.

The starting compound acetone oxime methyl ether is prepared by methodsknown from the literature, in particular by the process described inEP-A 708 082.

The cleavage is generally achieved by addition of aqueous hydrochloricacid having a concentration of at least 22%. In particular, the cleavageis carried out in hydrochloric acid having a concentration of from 22 to38% by weight, preferably from 30 to 38% by weight (concentratedhydrochloric acid). However, the cleavage can also be carried out bypassing gaseous hydrogen chloride into the ether. In this case, water isadded to the reaction mixture in an amount sufficient to prevent themethoxyamine hydrochloride formed from crystallizing out in the column.In general, 2 molar equivalents of water per mole of acetone oximemethyl ether used are sufficient for this purpose.

The hydrogen chloride introduced in the form of hydrochloric acid orgaseous hydrogen chloride is generally used in an excess over theacetone oxime methyl ether, but is used in at least the stoichiometricamount. The reaction is preferably carried out at a molar ratio ofhydrogen chloride to acetone oxime methyl ether of 2:1.

Particular preference is given to using a solution of acetone oximemethyl ether in aqueous hydrochloric acid.

Suitable reaction columns are, in particular, tray columns of all types.Bubble cap tray columns are particularly useful since the residencetimes on the trays can be readily set in them. Packed columns are alsopossible. The number of theoretical plates is restricted to 19 foreconomic reasons. The minimum number of theoretical plates necessary inthe column to guarantee complete conversion depends on the reflux ratioset at the top of the column. Since, as mentioned above, a reflux ratioof from 2 to 15 is preferably set, the minimum number of theoreticalplates in the reaction column is 8. On the other hand, if a reflux ratioof 16 or more is chosen, acetone oxime methyl ether can be cleavedcompletely even in reaction columns having less than 8 theoreticalplates.

The choice of the tray onto which the feed is introduced is demonstratedby way of example in the examples. It is generally chosen so that atleast 60% of the trays or theoretical plates are above the feed pointand at least 20% of the trays or theoretical plates are below the feedpoint. However, deviations can be made from this advantageous conditionsince the aim of the process can nevertheless be achieved by appropriateselection of the other parameters such as reflux ratio, temperature andamount taken off at the top.

The reaction temperature is generally in the range from 40 to 110° andpreferably from 65 to 95° C. and is determined, in particular, by theamount taken off at the top and the pressure which is set. The processis preferably carried out at a pressure of from 100 mbar to 3 bar.

The reactants acetone oxime methyl ether, water and hydrogen chloride oraqueous hydrochloric acid are advantageously fed into the middle sectionof the column. Feed rate and bottom heating power are generally selectedso that the mean residence time in the column is from 20 minutes to 2hours.

Furthermore, the cleavage can also be carried out in the presence of aninert organic solvent. The solvent is preferably recovered at the top ofthe column and should therefore have a boiling point of not more than150° C. If aqueous hydrochloric acid is used for the cleavage,particularly advantageous solvents are those such as toluene which forman azeotrope with aqueous hydrochloric acid.

Methoxyamine hydrochloride is preferably obtained at he bottom of thecolumn in the form of an aqueous suspension or solution which iscontinuously discharged, and the methoxyamine hydrochloride can beobtained in pure form by, for example, evaporation of the diluent orcrystallization. In all steps for the isolation of pure methoxyaminehydrochloride, a temperature of 60° C. should not be exceeded for safetyreasons. The aqueous solution of methoxyamine hydrochloride obtained ispreferably processed further without isolation of the methoxyaminehydrochloride. The yield is generally >98%.

Methoxyamine hydrochloride is an important intermediate in thepreparation of drugs and crop protection agents.

PREPARATION OF METHOXYAMINE HYDROCHLORIDE EXAMPLE 1

400 g/h of a solution of 26.1% by weight of acetone oxime methyl etherin 31% strength hydrochloric acid were ted continuously into a bubblecap tray column having 15 theoretical plates (corresponding to 25practical trays) and an internal diameter of 30 mm at the 11^(th)practical tray. The column was equipped with a thin film evaporator andan automatic runback divider.

The reflux ratio was fixed at 5. The bottom heating power was selectedso that the temperature on the 23^(rd) practical tray was >84° C. at apressure of 450 mbar at the top of the column. The temperature at thebottom of the column was 95° C. In this way, 280 g/h (maximum of 70% ofthe feed) of an aqueous hydrochloric acid solution of 35.1% by weight ofmethoxyamine hydrochloride (corresponding to a yield of 98.2%) could betaken off continuously at the bottom. The distillate obtained at the topcomprised mainly acetone together with water and dissolved hydrogenchloride.

EXAMPLE 2

500 g/h of a solution of 27.5% by weight of acetone oxime methyl etherin concentrated hydrochloric acid were fed continuously into a bubblecap tray column having 15 theoretical plates (corresponding to 25practical trays) and an internal diameter of 50 mm at the 11^(th)practical tray. The reflux ratio was fixed at 3. The bottom heatingpower was selected so that the temperature on the 22nd practical traywas >84° C. at a pressure of 450 mbar at the top of the column. Thetemperature at the bottom of the column was 98° C. In this way, 350 g/h(maximum of 70% of the feed) of an aqueous hydrochloric acid solution of37.5% by weight of methoxyamine hydrochloride (corresponding to a yieldof 99.5%) could be taken off continuously at the bottom. The distillateobtained at the top comprised mainly acetone together with water anddissolved hydrogen chloride.

EXAMPLE 3

The cleavage was carried out in a bubble cap tray column having 9theoretical plates at a reflux ratio of 11 under conditions otherwiseidentical to those in Example 1. A result comparable to that of Example1 was achieved.

EXAMPLE 4 (COMPARATIVE EXAMPLE)

500 g/h of a solution of 29.3% by weight of acetone oxime methyl etherin concentrated hydrochloric acid were fed continuously into a bubblecap tray column having 18 theoretical plates (corresponding to 30practical trays) and an internal diameter of 50 mm at the 16^(th)practical tray. The column was equipped with a thin film evaporator andan automatic runback divider. The reflux ratio was fixed at 4. Thebottom heating power was selected so that the temperature on the 28^(th)practical tray was >80° C. at a pressure of 450 mbar at the top of thecolumn. The temperature at the bottom of the column was 97° C. In thisway, 380 g/h (76% of the feed) of an aqueous hydrochloric acid solutionof 32.6% by weight of methoxyamine hydrochloride (corresponding to ayield of 88.1%) could be taken off continuously at the bottom. Thedistillate obtained at the top comprised mainly acetone together withwater and dissolved hydrogen chloride. Appreciable contents of unreactedacetone oxime methyl ether were found in the outputs from the top andbottom (3.4 and 3.5% by weight, respectively).

We claim:
 1. A process for the continuous preparation of methoxyaminehydrochloride by cleavage of acetone oxime methyl ether by means ofhydrogen chloride and water, wherein the cleavage is carried out in areaction column having less than 20 theoretical plates and the amounttaken off at the top is set to at least 30% of the amount of feed, andhydrogen chloride and water means a hydrochloric acid having aconcentration of at least 22% by weight.
 2. A process as claimed inclaim 1, wherein the hydrogen chloride and water are used in the form ofaqueous hydrochloric acid having a concentration of from 22 to 38% byweight.
 3. A process as claimed in claim 1, wherein an internaltemperature of from 40 to 110° C. is set.